Energy resource is the basis for the development of the current society, and is also an index for restricting the development of national economy and evaluating the national comprehensive strength. It is therefore important for the national safety and is always the first strategic field developed by the countries in the world. Up to date, the traditional fossil fuels, such as coal, oil and natural gas, suffer from low utilization rate, serious environmental pollution and shortage, and thus will not satisfy the requirement in future for an energy system that needs highly efficient, clean, economy and safe. The development of sustainable energy source is a huge challenge and under great pressure in both scientific and industry community. Meanwhile, people's concerns on the global environmental problems such as climate change and environmental pollution make the future energy production and utilization focus on the environmental and ecologic effect. The development of renewable energy solutions must be one of the important issues in the century.
Hydrogen energy is considered as an important secondary energy in the 21st century. It is a superior new energy source with main advantages as follows. Hydrogen has a high combustion heat value, and the combustion heat of every kilogram of hydrogen is about 3 times to that of petroleum, 3.9 times to that of alcohol and 4.5 times to that of char. The burning product of hydrogen is water, and thus it is indeed a clean energy. Hydrogen has abundant resources, and it could be obtained from water or biomass, both of them are the most abundant resource on earth, which is well evidenced by a classic procedure of nature in a recycling and sustainable way. The application of hydrogen energy is quickly growing with the progress of various techniques, and thus it is of great interest to develop hydrogen energy.
Nowadays, the production of hydrogen from fossil fuels is the main approach to obtain hydrogen in industry. As an alternative, the production of hydrogen by electrolysis is the other one. Although the current techniques for producing hydrogen from fossil fuels is mature with low production cost, the resources is limited and is non-renewable. In the long run, it does not meet the requirements for sustainable development. It is of great interest to produce hydrogen by solar energy, which converts the inexhaustible and dispersed solar energy into highly concentrated clean hydrogen energy. Currently, the methods for producing hydrogen by water-splitting with solar energy include a method for producing hydrogen by pyrolysing water with solar energy, a method for producing hydrogen by electrolysing water with solar powered electricity, a method for producing hydrogen by splitting water with solar energy, a method for producing hydrogen by photocatalytically reforming biomass with solar energy, and so on. It is of great significance in reality to produce hydrogen with solar energy, which, however, is a challengable research project with a large amount of theoretical and technical problems to be solved. It has drawn a great attention in the world, and many labor, funds and material resources have been invested. Particularly, the production of hydrogen with biomass, as an effective approach for solving energy problem, has recently appeared at a forefront.
Biomass is the most abundant material on earth, and includes all animals, plants and microorganisms as well as many organics derived, excreted and metabolised from the living materials. All the biomass has energy. Biomass energy is the energy produced from biomass using biomass as the carrier. The biomass energy is an energy form of solar energy that is stored in organism in the form of chemical energy, and comes from photosynthesis of plants, either directly or indirectly. Although the overall energy consumed by photosynthesis of plants only accounts for 0.2% of the total solar energy reaching on earth, the absolute value is surprisingly huge and the energy generated by photosynthesis is about 40 times to the total energy that human have consumed. Thus, biomass refers to a huge energy source. Yet, the biomass energy has clear disadvantages of low energy density and random distribution, although biomass is distributed all over the world with great reserves. As mentioned above, hydrogen is an excellent clean energy with high energy density and high efficiency. Hydrogen could be transported, or stored in long term, and liquified hydrogen has a density higher than natural gas. Thus, it would be easier to convert the large amount of dispersed biomass into hydrogen to transport and store, which is an important approach to store and concentrate biomass energy. More importantly, the technique of photocatalytically reforming biomass to generate hydrogen can be carried out under ambient temperature and pressure powered by solar energy. The fact for this energy conversion converts the inexhaustible solar energy into human desired energy is not only sustainable, but also environmental-friendly.
Thus, it is of great significance to develop a method for hydrogen production by solar energy with high efficiency and low cost, with respect to the improvement of energy infrastructure, the protection of ecological environment and the promotion of the sustainable development of economy and society.
Early in 1980s, Japanese scientists Kawai et al (Chem. Lett. 1981, 81-84; Nature. 1980, 286, 474-476) reported hydrogen generation by photocatalytically reforming biomass derivatives in water, using Pt/RuO2/TiO2 as the catalyst. Later, many literatures (J. Phys. Chem. 1983, 87, 801-805; J. Am. Chem. Soc. 1985, 107, 1773-1774; Chem. Phys. Lett. 1981, 80, 341-344; Photochem. Reviews 2003, 4, 5-18; Catal. Lett. 2004, 98, 61; Chem. Commun 2004, 2192-2193) reported methods for hydrogen generation from various biomass derivatives, such as methanol, ethanol, lactic acid, glycine, glutamic acid, proline, sugar, soluble starch, gelatin protein, algae, cockroach body, human urine, animal manure, teared filter paper (the major component thereof is cellulose), and so on.
Meanwhile, there are also patents involving photocatalytically reforming biomass derivatives by solar energy to generate hydrogen. Japanese patent application 57,156,302 disclosed a method for photocatalytically reforming methanol using TiO2/CdS/GaP to produce hydrogen, and Japanese patent application 59,203,701 disclosed a method for photocatalyticlly reforming a mixed solution of methanol and water with a ratio of 1:1 to produce hydrogen, which uses TiO2 loaded with one selected from a group consisting of CrB, Ni2B, Co2P, Mo2C and Cr3C2 as a catalyst. The rate of hydrogen generation is about 0.28 to 0.96 mL/h under irradiation of a 500 W UV lamp. Japanese patent application 6,186,943 disclosed a method for photocatalytically reforming a mixed solution of ethanol and water with a ratio of 1:1 to produce hydrogen, which uses amorphous Si loaded with Pt as a catalyst. The rate of hydrogen generation is up to 0.03 mL/h under irradiation of a 100 W halogen lamp. In addition, Can Li et al from DaLian Chemical and Physical institute, Chinese Academy of Sciences reported three different catalysts for photocatalytically reforming biomass derivatives to generate hydrogen. Chinese patent application CN200410031517.3 discloses a novel composite photocatalyst with a formula of A1-xTaO3:Bx, wherein x is 0 or 1, A is an alkaline metal and B is lanthanum or bismuth, for photocatalytically reforming biomass derivatives under UV light to generate hydrogen, and preparation method thereof. Chinese patent application CN200810240366.0 disclosed a photocatalyst based on heterojunction with a formula of m % WOxSy/CdS, wherein x represents the molar fraction of oxygen in the tungsten species with 0≦x≦1, y represents the molar fraction of sulfur in the tungsten species with 0<y≦2, and m is the mass percentage of tungsten element with 0<m≦10. The photocatalyst is based on semiconductor heterojuction, and a heterojunction photocatalyst with high activity for reforming biomass derivatives to generate hydrogen is prepared by using a CdS catalyst as a carrier, supporting a precursor of W on the CdS catalyst by impregnation, and then assembling the sulfide (oxide) of W on the CdS. Chinese patent application 200910136643.8 disclosed a TiO2 photocatalyst for photocataltically reforming biomass derivatives to generate hydrogen, wherein the crystal phase composition of anatase phase and rutile phase can vary in a relatively wide range. The TiO2 photocatalyst may be used in photocataltic reformation of biomass derivatives to generate hydrogen, largely improving the activity for hydrogen generation and effectively restraining the generation of carbon monoxide. The activity of the TiO2 photocatalyst for hydrogen generation is about five times to that of P25 as TiO2 control, and the content of CO in hydrogen is reduced at least two orders of magnitude to less than 5 ppm.
Up to date, however, no literature or patent has reported a high efficient, stable, low cost and easily made photocatalyst for photocatalytically reforming biomass derivatives to generate hydrogen, which can be synthesized in situ by a photochemical method from quantum dots and the salts or complexes of transition metals, particularly cheap transition metals such as nickel, cobalt and iron under mild conditions.